Ethylene-based resins are formed by various molding methods and used in various applications. Characteristics required of ethylene-based resins vary depending on their molding method and applications. For example, when a cast film is to be formed in T-die molding, there arises neck-in wherein the edge of the film is shrunk in a direction toward the center. When neck-in occurs, the film width is decreased and simultaneously the film edge becomes thicker than the film center, thus reducing the product yield. To minimize neck-in, an ethylene-based resin having a high melt tension for the molecular weight should be selected. This property is also necessary for preventing sagging or rupture in blow molding or for preventing bubble tremble or rupture in a process of inflation film.
Further, it is known that when a cast film is to be formed in T-die molding, regular variation in thickness occurring in the take-off direction of the film occurs, which is called “take-off surging”, which may also be called “draw resonance.” Take-off surging produces thickness variation in the film. As a result, the mechanical strength varies depending on the location. For this reason, in order to produce a film homogeneous in thickness stably, it is necessary to avoid the occurrence of take-off surging. In order to control the occurrence of take-off surging, it is believed that a resin property such that the rate of strain hardening of elongational viscosity increases as the rate of strain increases is required (for example, Toshitaka KANAI, Akira FUNAKI, Sen-i Gakkaishi (Journal of The Society of Fiber Science and Technology, Japan), 41, T-1 (1986)).
An already-known ethylene-based polymer obtained by using a metallocene catalyst is excellent in mechanical strength such as tensile strength, tear strength or impact resistance, but is inferior in melt tension, and thus exhibits significant neck-in. Further, take-off surging will be generated because the elongational viscosity does not show strain rate hardening property.
High pressure low-density polyethylene is superior in moldability regarding such as neck-in because it has a melt tension which is greater than that of an ethylene-based polymer obtained using a metallocene catalyst. Further, no take-off surging will be generated because the elongational viscosity shows strain rate hardening property. However, a high-pressure low-density polyethylene has a complicated long-chain branched structure, and thus is inferior in mechanical strength such as tensile strength, tear strength or impact strength.
As an ethylene-based polymer which maintains moldability regarding such as neck-in and take-off surging and has mechanical strength, a composition comprising an ethylene-based polymer obtained by using a metallocene catalyst and a high-pressure low-density polyethylene is proposed in, for example, Japanese Patent Applications Laid-Open No. H6-65443 and H7-26079, etc. When the content of high-pressure low-density polyethylene is high, however, the composition is expected to be inferior in mechanical strength such as tensile strength, tear strength or impact resistance. When the content of high-pressure low-density polyethylene is low, the composition does not sufficiently improve in melt tension, and is thus expected to exhibit deterioration in moldability, such as occurrence of significant neck-in.
To solve the problem, various ethylene-based polymers having long branches introduced into them by means of a metallocene catalyst have been disclosed. Japanese Patent Application Laid-Open No. H2-276807 discloses an ethylene-based polymer obtained by solution polymerization in the presence of a catalyst consisting of ethylene bis(indenyl)hafnium dichloride and methyl alumoxane, Japanese Patent Application Laid-Open No. H4-213309 discloses an ethylene-based polymer obtained by gaseous phase polymerization in the presence of a catalyst comprising ethylene bis(indenyl)zirconium dichloride and methyl alumoxane carried onsilica, WO93/08221 discloses an ethylene-based polymer obtained by solution polymerization in the presence of a constraint geometrical catalyst, Japanese Patent Application Laid-Open No. 8-311260 discloses an ethylene-based polymer obtained by gaseous phase polymerization in the presence of a catalyst comprising a racemate and mesoisomer of Me2Si (2-Me-Ind)2 carried on silica and methyl alumoxane. It is described that these ethylene-based polymers, as compared with long branch-free linear ethylene-based polymers, are excellent in moldability with improvement in melt tension, but neck-in is still significant so improvement in moldability is expected to be insufficient. Unlike high-pressure low-densitypolyethylene, such ethylene-based polymers do not show strain rate hardening property with respect to elongational viscosity. Therefore, improvement in take-off surging cannot be expected.
As described above, it was difficult to effectively obtain ethylene-based resin excellent in moldability, e.g. regarding neck-in and take-off surging, and mechanical strength by using conventional technologies.
The present inventors have made extensive study in view of these circumstances, and as a result, we have found that an ethylene-based polymer which generates less neck-in in T-die molding and no take-off surging and which is excellent in mechanical strength is obtained by imparting a specific molecular structure and melt physical properties thereto, and the present invention has been thereby completed.